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These are teleprompters, or more accurately the upper mirror of a free standing teleprompter. The screen is at the base of the device and can be larger and brighter than would otherwise fit on a small stand. They place the screen at a 45 degree angle and then place the mirror at a 45 degree angle such that the image looks like the screen is hovering directly in front of the speaker.

Teleprompters are actually used for far more than political presentations, but typically if the speaker is not presenting to a live audience, they will be presenting to a camera. For camera setups, they actually use a half-silvered (see-through) mirror that will project the text directly in front of the camera lens. This allows the speaker (such as a news caster) to read a script directly off the camera as they are looking at it.

In the case of live presentations however, they need to be able to engage the audience, which means they need to look around but still have the script close at hand. Using multiple teleprompters allows for this.